Considerable effort has been expended over many years to develop reduction/smelting processes that could replace blast furnaces for the production of liquid smelting iron, especially within the framework of small volume production units, and which avoid the preparation of materials, in other words, using ore fines and coal directly. Processes of this type are interesting since, in principle, installations involving a considerable investment, such as installations for producing coke and installations for ore agglomeration, can be avoided.
Direct reduction processes (without going through a liquid phase) using coal as a reducing agent are the most economical, particularly in countries without natural gas resources. However, a disadvantage of these processes is that they produce a pre-reduced iron ore with a high sulphur content (0.3–0.6% S by weight).
Amongst these processes, those using ore in the form of fine particles (fluid bed or multiple hearth furnace technologies) are especially interesting because they involve the least onerous form of ore. The particles of pre-reduced iron ore, also obtained in the form of fines, may be used without any difficulties in electric furnaces for producing steel, using cold or low temperature (<300° C.) blast injection procedures.
However, the massive use of this type of pre-reduced iron ore particles in steel producing electric furnaces poses two problems: it introduces a lot of sulphur, which is not eliminated in the oxidising metallurgical environment of steel producing electric furnaces, and it reduces the productivity of the electric furnace since their reduction—smelting from cold consumes more energy than that consumed by the principal raw material, scrap iron. This leads to over consumption of energy and, as a consequence, a loss of productivity.
These disadvantages can be avoided by producing smelting iron instead of steel. In fact, by directly introducing the particles of pre-reduced iron ore (the pre-reduced fines) from the reduction furnace, at a temperature of around 1000° C., into an electric furnace producing smelting iron, it is possible to get rid of the sulphur. In fact, feeding particles of pre-reduced iron ore into the furnace at 1000° C. considerably reduces the energy required for smelting. The production of smelting iron requires a reductive medium that makes it possible to reduce the amount of sulphur by nearly 90%. By creating a suitable slag, it is possible to obtain smelting iron with a sulphur content of 0.03–0.06%, which corresponds to a standard grade of smelting iron, which may then be used in all of the traditional uses of smelting iron, and in particular as a source of pure iron in electric furnaces.
All of this is true, particularly for the treatment, by reduction, of waste in the form of fines, which always gives a pre-reduced iron ore with a very high sulphur content. In the following description, “metallic fines” will be understood to mean all types of products containing partially oxidised metallic iron. The metallic fines represent particles of iron ore, all types of particles of waste containing partially oxidised iron and particularly fine particles from filters of blast furnace and electric furnaces, mill scale slivers or particles (iron oxides formed while re-heating or rolling), rolling or machining tailings, etc.
This type of smelting of fine metallic particles for the production of smelting iron is traditionally carried out in a resistance heating slag furnace, incorrectly called a submerged arc furnace (SAF). The fines are generally introduced into this type of electric furnace cold, by means of gravity. However, this type of electric furnace has limited power. In fact, the power density of a submerged arc furnace (SAF), expressed in MW/m2, is less, by a factor of five, than that of a free arc furnace. In order to obtain equivalent production levels, a submerged arc furnace with a diameter more than two times larger than that of an arc furnace has to be used.
In addition, in electric arc furnaces, the smelting of non-injectable finely divided materials leads to the formation of agglomerates, which are commonly called linings or berms, which stick to the walls. This is also the case during the smelting of finely ground scrap, turnings, millings, etc. The over-use of these materials obstructs part of the volume of the vessel, preventing correct introduction of the scrap, and the operator has to regularly carry out cleaning smelts by considerably overheating the furnace, which explains the loss of energy and production. As a consequence, the introduction by gravity of pre-reduced metallic fines into the electric furnace without taking any special precautions will inevitably lead to accretions and the formation of linings.
Under normal conditions of electric arc furnace operation, a foaming slag is used; in traditional smelting of scrap iron, the foaming of the slag is obtained by jointly blast injecting carbon and oxygen in order to form CO gas in the slag. When pre-reduced material rich in carbon (>2% C) is used, this foaming of the slag is spontaneous, since the pre-reduced iron ore provides both oxygen and carbon. Due to its low density and its thermal insulating properties, the foaming slag acts as an obstacle to the dissolution of the pre-reduced fines. The pre-reduced fines falling on the slag rapidly agglomerate and form a solid mass that is difficult to smelt, since it is not very dense, and leads to linings on the walls.
To produce smelting iron, carbon has to be used. Obviously, it is possible to inject carbon separately but the optimal method, in economic terms, consists in manufacturing a pre-reduced iron ore with an excess of carbon. This excess of carbon can be in a low proportion linked to the iron. However, when pre-reduced fines with 5–10% C are produced for manufacturing smelting iron, this carbon corresponds mainly to particles of free carbon. However, it is difficult to introduce this free carbon into the metal unless it is injected into the melt. In fact, the free arc electric furnace (unlike the submerged arc furnace, which in fact functions without an arc, by resistance heating) operates in a mainly oxidising atmosphere, in which the carbon oxidises rapidly. If no special precautions are taken, the input of non-injected carbon will mainly be lost in the gases, and the metal will become impoverished in carbon, and will give therefore a steel.
It would be advantageous to have an optimised process that makes it possible to produce smelting iron directly from particles of pre-reduced metallic fines in an electric arc furnace.